Fix SQLThresholdCheckOperator error on falsey vals

[Don-Burns]

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Summary

As documented in #37089 the SQLThresholdCheckOperator will currently fail if a "falsey" value is returned from the query (e.g. 0 or "").
This is a fix to change the error check a bit and as side effect make the exception for The following query returned zero rows:...
Whereas before, if no rows really were returned, the user would see the following:

result = hook.get_first(self.sql)[0]
TypeError: 'NoneType' object is not subscriptable

For testing, I used a parameterized test on an existing test and added a new test to cover the "no rows returned" case.
But please let me know if something else would be preferable.

closes: #37089

[boring-cyborg]

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[boring-cyborg]

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